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03-12-2012, 04:52 PM
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Join Date: Sep 2010
Location: Charlotte NC
Posts: 1,166
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One thing to consider in this discussion. Virtually every aircraft over times sees increases in the allowed gross weight. Often even on certified aircraft this is done without any other changes. As the aircraft flies over time and data is gained the weight is increased. This even applies to airliners and military aircraft. As I mentioned before there is a tremendous amount of data over time to support the higher weights on some of Vans aircraft. Delta recently upped the gross weights on all its 767ER's without any airframe changes. Just a note from Boeing saying its ok. The increased weight is used to takeoff for cross country flights and burned off before landing. I don't know anyone going out and loading a RVXX beyond the vans weights an launching directly into acro ect..
George
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03-12-2012, 05:04 PM
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Join Date: Apr 2005
Location: KSLC
Posts: 4,021
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Quote:
Originally Posted by sailvi767
I don't know anyone going out and loading a RVXX beyond the vans weights an launching directly into acro ect..
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Even though my RV6A GW was increased from 1650 to 1850, the aerobatic weights have remained the same as Van's listings. It seems that most others do the same.
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03-12-2012, 05:31 PM
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Join Date: Jan 2005
Location: SC
Posts: 12,887
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Quote:
Originally Posted by sailvi767
...I don't know anyone going out and loading a RVXX beyond the vans weights an launching directly into acro ect..
George
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Oh I know of at least one that does this...
__________________
Bill R.
RV-9 (Yes, it's a dragon tail)
O-360 w/ dual P-mags
Build the plane you want, not the plane others want you to build!
SC86 - Easley, SC
www.repucci.com/bill/baf.html
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03-12-2012, 06:50 PM
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Join Date: Oct 2009
Location: Victoria, BC, Canada
Posts: 3,932
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Quote:
Originally Posted by L.Adamson
On the other hand, Van gave Jon Johanson a one time approval for a gross weight increase of 136% on his stock built RV4 with tank modifications. This was required because it was built in Australia. At a normal gross of 1500 lbs, that would be 2040 lbs. Seems he's flown it around the world in different directions, at least three times.....perhaps more.
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This is true. However, i'm sure it came with restrictions. Jon had to be very careful what conditions he was flying in right after takeoff. If it was really gusty, I doubt it would have been all that safe. And while his gross may be "on the books" at 2000lb, I doubt he makes a habit of flying it anywhere near that weight.
If you have a loading scale that explains limitations vs. G-loading, like normal category 1800, utility 1600, aerobatic 1400, I could see that. But you haven't suggested that any such thing is necessary. Your -6 is registered at 1800, right? What G-load would you take your -6 to at that weight?
__________________
Rob Prior
1996 RV-6 "Tweety" C-FRBP (formerly N196RV)
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03-12-2012, 07:45 PM
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Join Date: Apr 2005
Location: KSLC
Posts: 4,021
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Quote:
Originally Posted by Snowflake
Your -6 is registered at 1800, right? What G-load would you take your -6 to at that weight?
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You'd have to know what an actual 6 wing tested to..............to really answer that question, wouldn't you?
Those 6 wings, the ones that attach in the middle. Amazingly strong!
L.Adamson --- 1850#
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03-13-2012, 07:46 AM
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Join Date: Oct 2009
Location: Victoria, BC, Canada
Posts: 3,932
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Quote:
Originally Posted by L.Adamson
You'd have to know what an actual 6 wing tested to..............to really answer that question, wouldn't you?
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No, I'm just asking what *your* operational limit is at *your* gross weight in *your* airplane. Do you know? Will you share?
Beyond the wing, there are other structures on the aircraft that become limiting factors at extreme load limits. The forces on your horizontal stab and elevator, for example, are directly proportional to the gross weight and max G loading. Van did change the skin thicknesses in the tail at some point in the RV-6... Not sure when though.
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Rob Prior
1996 RV-6 "Tweety" C-FRBP (formerly N196RV)
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03-13-2012, 08:11 AM
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Join Date: Feb 2007
Location: Mpumalanga, South Africa
Posts: 1,065
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I know this subject has been done to death and has drifted a little from the original discussion. However, I find it interesting that there seems more support for NOT increasing gross over recommended weights than is normal.
There seems to be 3 issues:
PERFORMANCE - We all know that VANS aircraft perform remarkably. Unless you are very hot and high, the performance at higher weights is unlikely to be a problem. Unfortunately, this adequate performance at higher gross may persuade some people that it is OK without considering other factors.
STRUCTURE - VANS tests the aircraft at the appropriate weight and G using a 50% safety factor. The wing must support this without permanent deformation. (What I don't know is if they go further to see when it ultimately fails, but no-one designs an aircraft heavier or stronger than it needs to be or the performance/payload suffers.) So, if you increase your gross by 10-15%, it is unlikely that pulling the G limit at the higher weight is going to cause failure which is why we don't hear of over-gross aircraft falling out of the sky every day. Again, this fact is going to persuade people that being over-gross is OK given the "evidence". What you are doing, of course, is eating into your 50% safety factor. So what happens on the day that you get airborne, meet someone joining the circuit the wrong way and over-G the aeroplane avoiding him? Now the safety factor has gone. Probably will never happen.......... The other issue is long-term fatigue and wear & tear at the higher weights. Rivets working loose, cracks forming quicker than they might otherwise do. This is why the one-off approvals are just that. The pilot and authorities accept that, statistically, for the round-the-world trip or whatever, the aircraft is unlikely to fail or be subjected to long-term fatigue in the short term. Do it consistently, though, and the odds start mounting up against you.
BALANCE - No testing has been done by VANS on the validity of the CG limits at higher weights. Certainly for larger aircraft, the CG limits normally narrow at higher weights and you simply can't assume that the quoted limits apply and provide acceptable handling at over-gross weights. Again, anyone planning a one-off approval is probably going to test the aircraft incrementally and get some data.
Bottom line - my opinion - simply because an aircraft has adequate performance, seems to handle OK and doesn't fall out of the sky doesn't mean it's OK to arbitrarily increase the gross weight. The better option is to stop eating meat and potatoes 3 times a day, have some salad and make some visits to the gym - that's what I'm doing to improve the payload! And I am "only" around 200lb - I'm not over-weight, I am under-height 
__________________
Paul
Mercy Air, White River FAWV
RV-10 ZU-IIZ - "Zeus"
Building Bearhawk Bravo - RV-18 not available
2019 Donation Made
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03-13-2012, 10:04 AM
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Join Date: Apr 2008
Location: Lake Havasu City AZ
Posts: 2,393
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over gross
I believe Jon Johanson's RV4 has been in a museum for several years. Did he not have some fuel near the wingtips?? Fuel at or near the wingtips reduces bending moment on the wings. One certified example of this is the Piper Twin Commanche with optional wingtip tanks of 30 gallons total. Gross weight with tip fuel is 125# greater than standard, so the tip fuel ALMOST gets a free ride.
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03-13-2012, 03:19 PM
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Join Date: Jan 2005
Location: Louisville, Ga
Posts: 7,840
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Yep.
Jon had 18 gallons in each wingtip, IIRC. My Cessna 310 had 55 gallons in each tip for the reasons you mention.
A couple of downsides to wingtip fuel is a dutch roll tendency, until you get used to handling it..and some airplanes are placarded against spins because of the high moment of inertia with that much fuel at the tips...spins may be unrecoverable 
__________________
Pierre Smith
RV-10, 510 TT
RV6A (Sojourner) 180 HP, Catto 3 Bl (502Hrs), gone...and already missed
Air Tractor AT 502B PT 6-15 Sold
Air Tractor 402 PT-6-20 Sold
EAA Flight Advisor/CFI/Tech Counselor
Louisville, Ga
It's never skill or craftsmanship that completes airplanes, it's the will to do so,
Patrick Kenny, EAA 275132
Dues gladly paid!
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03-13-2012, 07:19 PM
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Join Date: Apr 2005
Location: KSLC
Posts: 4,021
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Quote:
Originally Posted by paul330
I know this subject has been done to death and has drifted a little from the original discussion. However, I find it interesting that there seems more support for NOT increasing gross over recommended weights than is normal.
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Most likely, because the same subject pops up every few years. Between this forum, and the Matronics, which I used to use...............it's been discussed a lot. It looks like many who use to care, to respond, are not going to bother anymore.
L.Adamson
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